The weighed mean of the area and a size-class distribution of the glomeruli were calculated with a computer-assisted planimeter in a series of autopsy kidney specimens in Pima Indians affected by diabetes mellitus (DM) and in non-diabetic Pima Indians. These morphometric variables were also related to the severity of histologic glomerular lesions, graded on a four class scale. Glomerular size did not differ in the two groups, nor were differences in the size distribution discernible among the classes of histologic lesion. A similar pattern is found in the caucasian population affected by type-II DM, whereas type-I DM is associated with an early phase of increase and a late phase of decrease in glomerular size. The glomeruli of diabetic Pima Indians did not decrease in size even at the stage of diffuse sclerosis, a finding that contrasts with what is commonly seen in vascular nephropathy. The NIDDK epidemiologic survey of Pima Indians allows recognition of DM early after onset and long term follow up of the disease and its nephropathy. This offers a unique opportunity to study the natural history of diabetic nephropathy in a population with an incidence of type-II DM approaching 50% of the adults and an unusually young age of onset of the disease.